1. Field of the Invention
The present invention relates generally to a filter plate of the type utilized in a filter press for separating solids from liquids and particularly to a molded thermoplastic filter plate.
2. State of the Art
Filter presses are well known for separating solids from liquid slurries and the like. Conventional filter presses typically include a series of parallel, vertically disposed filter plates which are supported on a frame. Each filter plate is covered on both of its sides with a filtration medium or cloth which permits liquid to flow therethrough but which prevents the flow of solids contained in the liquid.
One type of filter plate utilized in such filter presses has a peripheral rim which extends completely around the perimeter of a recessed inner web such that inner recessed drainage faces are formed on both sides of the inner web. Thus, when adjacent filter plates are brought together in abutting relationship, the rims of adjacent filter plates engage. The filtration medium between the abutting adjacent plates serves as a gasket to form a liquid-tight seal around feed chambers formed between the recessed inner drainage faces of the adjacent plates.
For filtration, the individual filter plates of the filter press are brought together in abutting, sealing relationship by a closing device, typically a hydraulic ram. The liquid slurry or the like from which solids are to be removed is then fed under pressure into the chambers between adjacent plates. The pressure of the feed slurry forces the liquid to flow from the feed chambers through the filtration medium to the inner drainage faces of the plates. The solids in the slurry are left behind in the chambers between the plates. Filtration proceeds until the chambers are filled with filter cake or until a predetermined terminal pressure is reached. The filtration cycle is then complete; the feed to the press is stopped, the filter plates are moved apart and the filter cake is removed from the press.
In the past, filter plates of the type described above have been molded from thermoplastic materials such as, for example, polypropylene. Typically, such polypropylene plates have raised drainage ribs molded on the inner recessed drainage faces of the plate. These raised drainage ribs serve to hold the filtration medium or cloth above the surface of the inner drainage face so that filtered liquid or filtrate which has passed through the filtration medium can flow along the surface of the inner drainage face to filtrate discharge holes formed in the rim of the plate. Molded thermoplastic filter plates found in the prior art utilize drainage ribs which are formed primarily in a single direction across the inner drainage face of the plate. It has been found that molded filter plates having this type of unidirectional rib structure shrink in the direction of the ribs while the filter plate is setting up in the mold. This shrinkage creates high stress areas over the surface of the filter plate and, thus, reduces its strength.